I'm really worried about my old friend?

We're 15 years old and she moved to a new school 2 months ago and I sometimes see her on the bus and hear lots of gossip about her. She is a party girl, she used to have lots of abuse from living with her mom and step dad but it got better when she moved in with her dad... When I hung out with her a lot, she only smoked weed and did a few party drugs when she went to raves and stuff but now I'm hearing she is a major drug dealer/druggie/even prostitute! When I see her on the bus it doesn't even look like she recognises me, she just keeps her head down with her hair over her face, looking dead. I also hear she is doing hard drugs, all of them, but mostly heroin. Some people who meet up with her sometimes to chill noticed track marks on her hands/arms and harsh abrasions on her body from getting "way too messed up". She has a new group of friends too at this alternative school she is going to, with someone I know for a fact does crack and another guy who does inexplicable amounts of drugs that he has hppd or schizophrenia- he can't even tell! To be honest, I AM really fricken scared of her now, but I'm so scared that she's going to die from an overdose or something. She's lost a lot of weight too, she looks about 105lbs and she's like 5'5. A lot of guys like her though, she's quite well-known. What should I do? Should I just let her do her own thing? We haven't talked much since she left, but I see her quite often because her school is 15 blocks farther than mine and we take the same bus.